(1) Field of the Invention
The present invention relates to a ball-bearing control cable, to a mechanical system provided with such a ball-bearing control cable, and to an aircraft including such a mechanical system.
The invention thus lies in the technical field of ball-bearing control cables. Such a ball-bearing control cable is also called a “ball-bearing sheath” or a “ball-bearing control” by the person skilled in the art.
(2) Description of Related Art
A mechanical system may comprise a ball-bearing control cable for transmitting traction or compression forces.
By way of example, aircraft sometimes have ball-bearing control cables in a movement transmission linkage for controlling a flight control member of the aircraft. By way of illustrative example, a helicopter may have servo-controls for controlling the pitch of the blades of the lift and propulsion rotor of the helicopter. Those servo-controls are thus controlled by movement transmission linkages that are connected to a stick referred to as the “cyclical” stick and to a lever referred to as the “collective pitch” lever. Those movement transmission linkages may thus comprise ball-bearing control cables.
A ball-bearing control cable usually comprises a movable central blade that is guided axially by balls inside an outer sheath, while enabling the central blade and the outer sheath to follow sinuous paths.
The central blade extends longitudinally between two end members that are to be connected together mechanically in order to transmit traction or compression forces from one member to the other member. Thus, the central blade extends longitudinally from a first end secured to a first end member to a second end secured to a second end member.
Furthermore, the central blade passes through the inside of the outer sheath in order to be connected to the first end member and to the second end member. By way of example, this outer sheath comprises a flexible metal tube made of metal and covered with a protection tube made of plastics material.
In addition, the central blade is movable relative to the outer sheath, while being guided axially by balls. Under such circumstances, the ball-bearing control cable comprises two rows of balls arranged in the outer sheath on either side of the central blade. Each row of balls usually has an elongate structure defining cages, each ball being arranged in a cage.
As a result, a first row of balls is arranged in the outer sheath between a top face of the central blade and a top plate. A second row of balls is arranged in the outer sheath between a bottom face of the central blade and a bottom plate. Furthermore, the balls can roll in grooves made in the bottom face and the top face of the central blade.
In addition, the ball-bearing control cable may include an endpiece interposed between the outer sheath and the central blade, in particular in order to keep the balls inside the outer sheath.
According to Document U.S. Pat. No. 5,722,301, such an endpiece comprises a tube extending the outer sheath. That tube extends longitudinally from the outer sheath towards a dropped edge clamped on a pipe. That pipe is disposed about one end of the central blade.
In addition, a ring arranged in the tube of the endpiece surrounds the central blade and blocks the rows of balls in longitudinal translation.
Such a ball-bearing control cable is sufficiently rigid longitudinally to allow a traction force or a compression force to be transmitted from one end member to another end member. In addition, the ball-bearing control cable presents transverse flexibility that is sufficient for following a sinuous path.
In this context, ball-bearing control cables are used in particular on a helicopter instead of rigid control rods. In particular, the ball-bearing control cables are sometimes interposed in a movement transmission linkage in order to control a hydraulic system and in particular a servo-control. Moving a flight control thus leads to a central blade of a ball-bearing control cable moving, with that movement of the central blade leading to the servo-control being operated.
Those ball-bearing control cables are nevertheless at risk of being subjected to destructive forces in the event of a member of the movement transmission linkage being prevented from moving. By way of example, an input lever of a servo-control may be prevented from moving in the absence of hydraulic assistance. The movement of a flight control thus leads to a traction or compression force being created that acts on the central blade. That force can generate a force in the outer sheath that might deteriorate said outer sheath.
Document FR 1 403 706 is remote from that problem. Document FR 1 403 706 describes a cage for spacing apart a rolling body.
Document FR 2 576 070 presents a ball-bearing control cable.
Documents FR 2 508 197, FR 366 246, DE 1 525 215, U.S. Pat. Nos. 2,369,332, 3,464,287, EP 0 933 541, FR 360 475, and DE 6 608 858 are also known.